I find that you give too much credits to pubbies, do you seriously think they'll all gang up the plate wearer? Nah, they'll all gang up the little peasant to get more and easy kills.
Also, depending on how the devs balance this shit if they ever implement it, I don't think it would screw cavs unless you went plated charged in black armor.
Honestly, I say the devs are more likely to screw cavalry several times over more than my suggestion.